The present invention is concerned, in particular, with a problem that arises in video projectors which use LEDs as the light source and a micromirror actuator as the imaging element. A micromirror actuator is a micromechanical component which, with the aid of individual movable mirrors can be used for controlled light deflection. Using a matrix-shaped arrangement, micromirror actuators can deflect the light of a strong light source, in this case LEDs, such that an image is projected. Designations under which this technology is to be found are Digital Micromirror Device (DMD) and Digital Light Processing (DLP).
The micromirror actuators usually comprise matrix-shaped arrangements of individual elements, the individual micromirrors comprising a tiltable reflective surface with an edge length of a few micrometers. The micromirrors on a DMD chip have, for example, an edge length of approximately 16 μm and are therefore smaller than a fifth of the width of a human hair. The movement is evoked by the force effect of electrostatic fields. Each micromirror can be adjusted individually with respect to the angle thereof and typically has two stable end states, between which said mirror can change up to 5000 times in a second.
DMD chips with an XGA image resolution of 1024×768 pixels contain an array of 786,432 minute mirrors. DMD chips with resolutions of up to 2048×1080 pixels are also now available.
Different brightness levels of the individual image points are generated with binary pulse-width modulated actuation. In order to represent, for example, 32 (=25) brightness levels, five states are required. Said states differ in how long the DMD is switched, i.e. on. In the first state (bit 0), the mirror is on or off (1 or 0) for the shortest possible time. In the next state (bit 1), the time is doubled, and so on. The total time for a cycle with 5 bits is therefore 496 μs.
In order to generate colored image points, in video projectors which function with LEDs as the light source, three LEDs are normally used, specifically one LED which emits red light, one LED which emits green light and one LED which emits blue light.
In a primitive solution, the image repetition frequency (frame rate) is 60 Hz and thus the frequency at which the three LEDs are operated is 3×60 Hz, which is 180 Hz. In order to avoid the rainbow effect, each image is repeated a plurality of times. Currently 16, 18 or 20 partial images per frame are usual. This results in an on-off frequency of the LEDs of 960, 1080 or 1200 Hz. Given a ⅓ on-time, the pulse lengths, i.e. the switch-on times per LED are therefore approximately 277 μs to 347 μs. Since the image processing algorithm involved is based on the assumption that a constant light amplitude prevails during the whole of each pulse length, then even transient phenomena of approximately 10 μs have a negative effect.
Whereas, when a lamp is used as a light source, few current variations occur because the lamp integrates the current with a time constant of approximately 100 μs, the problem arises, when using an LED as the light source, that the light emitted by the LED follows the driving current practically without delay. If the driving current contains AC components, that is, “ripple currents”, the consequence thereof is that image points which should, in principle, be equally bright, are actually displayed at different brightness levels. The alternating current component of the LED current which overlays the DC component of the LED current is designated the ripple current. Whereas at points with a high brightness level, the integrating capability of the human eye integrates mean value variations in the LED current and said variations are therefore rendered insignificant, the lower the brightness level of the image point to be displayed, the more critical said problem becomes. Since the image point is only briefly switched on, the integration capability of the human eye is of no use in this case. The eye now perceives brightness variations.
The relevant LED is therefore not always on, but only when the relevant color is needed to display the respective image point. As previously mentioned, the transient behavior of the respective color is therefore of particular significance. Short time periods are therefore desirable for the transition from a first level to a second level and, because of the aforementioned problem, the AC components of the current should be as small as possible within these time periods, i.e. the target value must be reached as fast as possible and without significant overshoot.
The use of linear controllers or unsynchronized switching regulators as drivers for the LEDs of a video projector, with DMDs as the imaging elements, is known. Linear controllers have the advantage of short rise times and a negligible ripple current or AC component. However, if the output voltage of a driver of this type is approximately 7 V, whilst LEDs usually have a forward voltage in the range of 3 V to 5 V, given a typical LED current of approximately 30 mA, a significant power loss is caused in the switch of the linear controller. This makes complex cooling measures necessary whilst also resulting in poorer efficiency.
Unsynchronized switching regulators, the current waveform from which is essentially triangular, have the advantage of high efficiency since the switch of the switching regulator is either on or off and therefore does not enter a semiconducting state as in the case of a linear controller. However, a compromise is always required between the rise time and the ripple current (the AC component). A short rise time implies a relatively large ripple current, whilst a small ripple current implies a long rise time. The disadvantages associated with a large ripple current have already been set out in detail above. In summary, the use of a linear controller and of an unsynchronized switching regulator therefore both leave problems unsolved.